Starting and operating circuit for arc discharge lamp

ABSTRACT

A circuit for starting and operating a discharge lamp, such as a compact fluorescent lamp, at high frequency from a 60 Hz AC supply. The circuit includes a DC power supply coupled to a pair of AC input terminals for generating a DC voltage. An oscillator coupled to the DC power supply includes a pair of semiconductor transistors and a drive transformer having a primary winding and at least one secondary winding. A load coupled to the output of the oscillator comprises a series combination of the primary winding of the drive transformer and a tank circuit including a tank inductor and a tank capacitor. Suitable means for connecting a discharge lamp in parallel with the tank capacitor is provided. An oscillator sensing and controlling circuit responsive to various lamp failure modes is coupled to the secondary winding of the transformer and reduces the output of the oscillator after a predetermined time delay by shunting the base of either one of the semiconductor switches of the oscillator. Upon replacement of the failed lamp, normal operation of the oscillator is resumed without having to disconnect the AC supply.

This is a continuation of copending application Ser. No. 07/749,814filed on Aug. 26, 1991 now abandoned.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application discloses and claims structural features for a startingand operating circuit which constitutes improvements over relatedsubject matter disclosed and claimed in U.S. patent application Ser. No.07/664,161 (now U.S. Pat. No. 5,138,235), filed Mar. 4, 1991 andassigned to the same assignee.

U.S. patent application Ser. No. 07/749,813 (now U.S. Pat. No.5,142,202), filed concurrently herewith and assigned to the sameassignee, contains related subject matter.

FIELD OF THE INVENTION

This invention relates to low-pressure discharge lamps, particularlyfluorescent lamps, and especially to starting and operating circuitryfor compact fluorescent lamps.

BACKGROUND OF THE INVENTION

Various types of operating circuits are known to start and operatecompact fluorescent lamps. One type of circuit is illustrated in theFIG. 7 schematic of French Publication No. 0346782. This schematic issimilar, in general principle, to the state of the art as practised in alamp sold by the Osram Company under the registered trademark "DULUX EL"or in the lamp of the Philips Company which bears the denomination "PLC20 Electronic". Using the circuit of French Publication 0346782 as anexample of such circuits, after the two input terminals of the DC/ACconverter (or oscillator) are energized by a DC voltage which appearsacross a filter capacitor, the starting capacitor C5 charges through astarting resistor R3 to a voltage which is substantially equal to thethreshold voltage of the threshold element (i.e., the diac). Thethreshold element breaks down and supplies a pulse to the base terminalof transistor T2. As a result, transistor T2 begins to conduct. Acurrent flows through transistor T2 and the load circuit. Subsequently,this transistor becomes non-conducting and the other transistor T1becomes conducting. This process is then continuously repeated. Thisleads to an oscillation, i.e., an alternating current through the loadcircuit which includes the discharge tube.

U.S. Pat. No. 5,138,235 describes some disadvantages which may appear insome circuits similar to those described above. For example, when powerto the circuit is removed, a momentary blink or flicker in the lamp mayoccur immediately after the tube is extinguished. It was observed thatwhen AC power to the circuit is removed, a voltage initially remains onthe filter capacitor of the DC power supply. This filter capacitorvoltage gradually depletes to a point (usually greater than the starterthreshold voltage) where the oscillator shuts down. However, thestarting capacitor is allowed to recharge to a point where the thresholdelement of the starting circuit triggers causing the oscillator toconduct for a short period of time. Consequently, the discharge tubewill blink or flicker as a result of current from the filter capacitorflowing through the conducting transistors and load circuit. Thisconduction continues for approximately 10 msecs. until the filtercapacitor voltage is less than the starter's trigger voltage.

Another problem, which may appear in circuits similar to those describedabove, is at the end-of-lamp life when the emissive material on one orboth of the filament electrodes has depleted. Although a discharge isunable to be established between the lamp electrodes, the oscillator maycontinue to conduct current through circuit components causing anunnecessary consumption of power until, for example, the AC power sourceis disconnected or the lamp and tank capacitor are removed from the loadcircuit. In the instances where the lamp is permanently connected to thecircuitry, such as in an integral lamp unit, this latter option isunavailable.

U.S. Pat. No. 5,138,235 proposes the use of fusible-type base driveresistors. As a result of the increased current flow caused by thenon-operable lamp, one of the fusible resistors creates an open circuitand thereby permanently inhibits operation of the oscillator.

While the use of fusible resistors to permanently inhibit the oscillatorupon end-of-lamp life is indeed effective, it may be undesirable toimplement this approach with a lamp unit containing a replaceable lampwithout having made other provisions. It is apparent that followingend-of-lamp life, the oscillator in a replaceable lamp unit must remainfunctional in order to accommodate a replacement lamp.

U.S. Pat. No. 4,554,487, which issued to Nilssen on Nov. 19, 1985,describes a subassembly which disables the inverter in case the inverteroutput power remains at an excessive magnitude for more than a verybrief period. The input of the subassembly includes a voltage-clampingdevice (e.g., a varistor) coupled in parallel with the tank capacitor ofa series-resonant LC circuit so as to limit the voltage developedthereacross. Also included in the subassembly is a current sensingcircuit for sensing current flow through the varistor. The output of thesubassembly is connected directly to the junction of transistor Q2 andthe secondary winding CT2s of positive feedback current transformer CT2.While such disabling circuitry may effectively prevent the inverter fromself-destructing in case the fluorescent lamp fails to start or if thelamp is removed from the circuit, several disadvantages still exist withthis approach. For example, the subassembly of Nilssen requires a highvoltage clamping device (e.g., a varistor) which may be relativelyexpensive. Moreover, once the inverter illustrated in FIG. 1 of Nilssenhas stopped oscillating, the inverter will not restart until power linevoltage is removed and then reapplied at a later time (i.e., after muchof the charge on the filter capacitors has had a chance to leak off). Itmay be advantageous to be able to replace a failed lamp without havingto remove the power line voltage.

Another approach is described in U.S. Pat. No. 4,644,228, which issuedto Nilssen on Feb. 17, 1987, wherein a control means provides a shortcircuit across the tank capacitor in the event the lamp fails to conductwithin about 25 milliseconds. After about 1.5 second, the short circuitis removed for about 25 milli-seconds, thereby permitting the voltageacross the tank capacitor to grow to a magnitude sufficient to igniteand operate the lamp. If lamp current does not then flow, or if at anytime it ceases to flow, the short will be reapplied within 25milli-seconds. Thereafter, until power is removed or until an operablelamp is connected, the control means will continuously repeat the cycleof 1.5 second short circuit and 25 milliseconds open circuit. While suchcontrol means, which periodically provide a short circuit across thetank capacitor, may operate effectively without requiring the removaland reapplication of power in order to replace a failed lamp, thecontrol circuit requires the use of high voltage components, such asvaristor V, bridge rectifier BR and transistor Qa in order to withstandthe high lamp starting voltage.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to obviate thedisadvantages of the prior art.

It is still another object of the invention to provide an improvedcircuit for starting and operating a discharge lamp which does notrequire a voltage limiting device in the oscillator disabling circuitry.

It is another object of the invention to provide an improved circuitwhich remains fully operable upon failure of the lamp and does notrequire removal of the power line voltage in order to replace a failedlamp.

It is still another object of the invention to provide an improvedcircuit having a protection circuit which does not require relativelyexpensive, high voltage components in the oscillator disablingcircuitry.

These objects are accomplished in one aspect of the invention by theprovision of a starting and operating circuit for a discharge lampcomprising a pair of AC input terminals adapted to receive an AC signalfrom an AC power supply and a DC power supply coupled to the AC inputterminals for generating a DC voltage. An oscillator coupled to the DCpower supply includes oscillator drive means including at least onesemiconductor switch and a transformer having a primary winding and asecondary winding. The secondary winding is coupled to the semiconductorswitch of the oscillator. Load means coupled to the output of theoscillator comprises a series combination of the primary winding of theoscillator drive transformer and a tank circuit including a tankinductor and a tank capacitor. Means is provided for connecting adischarge lamp in parallel with the tank capacitor. Oscillator sensingand controlling means is coupled to the secondary winding of theoscillator drive transformer for sensing the output of the oscillatorand then for reducing the output of the oscillator after a predeterminedtime delay following the initiation of a resonant mode condition of thetank circuit.

In accordance with further teachings of the present invention, theoscillator sensing and controlling means includes a series combinationof a diode and a semiconductor switch means coupled across the inputterminal of the semiconductor switch means of the oscillator.Preferably, the oscillator controlling means includes voltage rectifyingmeans in series with the charging resistor means in addition to meansfor controlling the duty cycle of the semiconductor switch means.

Additional objects, advantages and novel features of the invention willbe set forth in the description which follows, and in part will becomeapparent to those skilled in the art upon examination of the followingor may be learned by practice of the invention. The aforementionedobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combination particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily apparent from the followingexemplary description in connection with the accompanying FIGURE. ThisFIGURE represents a schematic diagram of a preferred embodiment of astarting and operating circuit for a discharge lamp according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above-described drawings.

The sole FIGURE represents a schematic diagram of a preferred embodimentof a starting and operating circuit for a discharge lamp LP1. Lamp LP1is an arc discharge lamp such as a low-pressure fluorescent lamp havinga pair of opposing filamentary electrodes E1, E2. Each of thefilamentary electrodes is coated during manufacturing with a quantity ofemissive material. Lamp LP1, which forms part of a load circuit 16, isignited and fed via an oscillator 12 which operates as a DC/ACconverter. Oscillator 12 receives filtered DC power from a DC powersupply 10 which is coupled to a source of AC power. Conduction ofoscillator 12 is initiated by a starting circuit 4. Upon detection of anabnormal increase in load current caused by operating the circuit with alamp which has an internally shorted coil or by operating the circuitwith a lamp having a depleted lamp coil or a cracked envelope, a voltagesensing and oscillator regulating circuit 20 temporarily shuntsoscillator drive current and thereby substantially reduces theoscillator output.

A pair of input terminals IN1, IN2 are connected to an AC power supplysuch as 108 to 132 volts, 60 Hz. A transient suppressor RV1 is shuntedacross input terminals IN1, IN2 in order to absorb any surge energy thatmay otherwise cause damage to the circuit. The AC input power is coupledby way of a fuse F1 to the input of DC power supply 10 which consists ofdiode rectifier bridge D3 and a filter capacitor C6. Capacitor C6filters the rectified AC voltage so that the bus voltage (VBUS) is a DCvoltage with minimal low frequency modulation which serves to minimizelamp current crest factor. A capacitor C5, which is connected inparallel with transient suppressor RV1 and the input to DC power supply10, and an inductor L2 connected to the positive output terminal of DCpower supply 10 serve to suppress EMI generated by oscillator 12.

Oscillator 12, which includes (as primary operating components) a pairof series-coupled semiconductor switches, such as bipolar transistorsQ1, Q2 or MOSFETS (not shown), is coupled in parallel with the output ofDC power supply 10. The collector of transistor Q2 is connected to oneend of inductor L2 while the emitter of transistor Q2 is connected toone end of a resistor R5. The other end of resistor R5 is connected tothe collector of transistor Q1. The emitter of transistor Q1 is coupledto circuit ground through a resistor R6. During lamp operation, emitterresistors R5 and R6 minimize lamp current variations caused bytemperature. More specifically, as the junction temperatures oftransistors Q1 and Q2 increase due to increases in ambient temperature,the base-emitter voltages of Q1 and Q2 tend to decrease. As a result,the voltage drop across resistors R5 and R6 increases and therebycompensates for the decrease in the base-emitter voltage. Consequently,the lamp current will remain relatively constant with temperature. Inaddition to compensating for temperature variations, resistor R6 acts tolimit the current through transistor Q1 at initial startup.

For transistor protection during starting, a series combination of adiode D4 and a zener diode D5 is connected across the base-emitterjunction of transistor Q2. Similarly, a series combination of a diode D6and a zener diode D7 is connected across the base-emitter junction oftransistor Q1.

Base drive and switching control for transistors Q1 and Q2 are providedby secondary windings W3, W2 of a saturable transformer T1, baseresistors R2 and R3, and capacitors C2 and C3, respectively. The valuesof resistors R2 and R3 are chosen so that transistor control or baseleads are properly driven.

Oscillator starting circuit 14 includes a series arrangement of aresistor R1 and a capacitor C1. The junction point between resistor R1and capacitor C1 is connected to a bidirectional threshold element D2(i.e., a diac). One end of threshold element D2 is coupled to the baseor input terminal of transistor Q1 through base resistor R2. Asillustrated in the FIGURE, the input of the oscillator starting circuit14 (i.e., the upper end of resistor R1) is connected to one of theterminals (e.g., IN2) applied to the AC power supply.

During normal lamp operation, oscillator starting circuit 14 is renderedinoperable due to a diode rectifier D1 connected to the junction pointof resistor R1 and capacitor C1. During lamp operation, diode rectifierD1 holds the voltage across starting capacitor C1 at a level which islower than the threshold voltage of threshold element D2. The timeconstant of R1 and C1 should be longer than the reciprocal of theoperating frequency of the oscillator in order to insure that capacitorC1 does not recharge during normal operation to the threshold voltage ofelement D2.

Series capacitors C9 and C10 form one of the two legs of the half-bridgetopology. The other leg being formed by the series coupled transistorsQ1 and Q2. Unlike conventional half-bridge circuits in which the twocapacitors are the main energy reservoirs, in the present circuitcapacitors C9 and C10 function as a voltage divider and help shuntEMI/RFI noise generated by transistors Q1 and Q2.

Load circuit 16 comprises a series combination of a primary winding W1of transformer T1 and an inductor L1 connected in series with a parallelcombination formed by capacitor C4 and lamp LP1 (when connected).Terminal 1 of inductor L1 is connected to terminal 2 of winding W1.Inductor L1 comprises the principle ballasting element for the lamp. Thesaturation of transformer T1 influences the switching frequency oftransistors Q1 and Q2. Typically, the transistor switching frequency isfrom about 25 Khz to 39 Khz. Preferably, the switching frequency isabout 30 Khz. During lamp operation, the impedance of capacitor C4 ismuch higher than the impedance of the lamp, so capacitor C4 acts as anopen circuit. The total load impedance is the sum of the impedance ofinductor L1 and the lamp impedance in series, which will make the lampcurrent a sawtooth waveform. The resonant frequency during normaloperation is very different from the resonant frequency during startup.In normal operation, the load circuit is in a critically damped mode.

Load circuit 16 further includes a capacitor C8, connected across thecircuit arrangement of primary winding W1, inductor L1, lamp LP1 (whenconnected) and capacitor C4. Capacitor C8 forms a single element snubbercircuit which reduces the rise time and thus the switching losses oftransistors Q1 and Q2. As a result of the reduction in rise time (orequivalent reduction in dV_(ce) /dt) of transistors Q1 and Q2, highvoltage spikes which normally generate EMI/RFI noise are reduced.

Preferably, the terminals of discharge lamp LP1 are connected to loadcircuit 16 by means of suitable sockets in order to facilitate lampreplacement. In the arrangement shown in the FIGURE, capacitor C4 isadapted to be coupled to terminal 3 of inductor L1 and to one end ofcapacitor C8 (and the junction of capacitors C9 and C10) throughelectrodes E1 and E2, respectively, when the lamp terminals areconnected to the socket terminals. Although the FIGURE illustrates thateach electrode E1, E2 is coupled via a pair of lamp terminals to a pairof socket terminals, other coupling arrangements are possible. Forexample, if lamp replacement is not desired, as in the case of anintegral lamp unit, lamp LP1 may be directly wired to the load circuit16. Also, as is conventional in instant-start fluorescent lamps, thelead-in wires from each cathode may be shorted together resulting in asingle terminal being available from each electrode. In the latter case,capacitor C4 may be directly connected to terminal 3 of inductor L1 andto capacitor C8 (at the junction of capacitors C9 and C10).

In accordance with the teachings of the present invention, the startingand operating circuit further includes means for temporarily reducingthe output of the oscillator upon detection of an abnormal lampcondition, such as the depletion of emissive material on one or more ofthe lamp electrodes or a cracked lamp envelope.

In a preferred embodiment, the starting and operating circuit includes avoltage sensing and oscillator regulating circuit 20. Upon detection ofan abnormal increase in load current as detected by an increase involtage across winding W3, circuit 20 causes conduction of asemiconductor switch Q3 in the oscillator regulating portion of circuit20 which causes a temporary shunting of oscillator drive current.

In the embodiment illustrated in the FIGURE, the input of circuit 20 isconnected to secondary winding W3 on transformer T1. Terminal 4 ofwinding W3 is connected to a voltage divider resistor R7. The other endof resistor R7 is connected to a voltage divider resistor R9 whose otherend is connected to terminal 3 of winding W3 which is also connected tocircuit ground. The junction of voltage divider resistors R7 and R9 isconnected to the anode of a diode D8. The cathode of diode D8 isconnected to one end of a capacitor C7. A discharge resistor R8 isconnected across the terminals of capacitor C7. The junction formed byresistor R8, capacitor C7 and the cathode of diode D8 is connected tothe gate (or input) terminal of a semiconductor switch Q3 whichpreferably consists of a MOSFET. Alternatively, semiconductor switch Q3may be, for example, an IGBT. The drain terminal of semiconductor switchQ3 is connected to the cathode of a diode D9. In order to provide arelatively low forward voltage drop (i.e., less than 0.5 volts) acrossthe base of transistor Q1 and ground, diode D9 is preferably a schottkyor germanium diode. The source terminal of semiconductor switch Q3 iscoupled to circuit ground.

Since the maximum voltage developed across winding W3 of transformer T1is only about 10 to 15 volts during lamp starting and 5 to 6 voltsduring normal operation, diode D9 and transistor Q3 have relatively lowvoltage ratings. Moreover, a voltage clamping device (i.e., a varistor)is not necessary.

According to the teachings of U.S. Pat. No. 5,138,235, base driveresistors R2 and R3 and emitter resistors R5 and R6 may be fusible typeresistors. As a result of the increased current flow in the oscillator,one of the fusible resistors creates an open circuit and therebypermanently inhibits operation of the oscillator.

Without having made other provisions, the use of fusible resistors topermanently inhibit the oscillator upon end-of-lamp life may beundesirable when used with a lamp unit containing a replaceable lamp.However, to provide added protection in the event that one of thecomponents of oscillator control circuitry should unexpectedly fail andrender this portion of the starting and operating circuit inoperable,the use of fusible resistors in accordance with the teachings of U.S.Pat. No. 5,138,235 is recommended.

The operation of the circuit will now be discussed. When terminals IN1and IN2 are connected to a suitable AC power source, DC power source 10rectifies and filters the AC signal and develops a DC voltage (VBUS)across capacitor C6. Simultaneously, during the negative half cycle ofthe AC input signal, starting capacitor C1 in oscillator startingcircuit 14 begins to charge through resistor R1 to a voltage which issubstantially equal to the threshold voltage of threshold element D2.Upon reaching the threshold voltage (e.g., 32 volts), the thresholdelement breaks down and supplies a pulse to the input or base terminalof transistor Q1. As a result, current from the VBUS supply flows tocircuit ground through inductor L2, capacitor C9, capacitor C4, ballastinductor L1, primary winding W1 of transformer T1, the collector-emitterjunction of transistor Q1 and emitter resistor R6. Since the lamp isessentially an open circuit during starting, no current flows throughthe lamp at this time. Current flowing through primary winding W1 causessaturation of the core of transformer T1 which forces the inductance ofthe transformer T1 to drop to zero. A resulting collapse in the magneticfield in transformer T1 causes a reverse in polarity on secondarywindings W2 and W3 of transformer T1. As a result, transistor Q1 isturned off and transistor Q2 is turned on. Current now flows to groundthrough inductor L2, the collector-emitter junction of transistor Q2,emitter resistor R5, primary winding W1 of transformer T1, ballastinductor L1, capacitors C4 and C10. This process is repeated causing ahigh voltage to be developed across capacitor C4 (and lamp LP1) as aresult of a series resonant circuit formed by capacitors C4, C9, C10 andballast inductor L1. The high voltage developed across capacitor C4 issufficient to ignite lamp LP1. In addition to igniting lamp LP1,capacitor C4 improves lamp current crest factor.

During normal lamp operation, oscillator starting circuit 14 is renderedinoperable due, in part, to rectifier D1 which holds the voltage acrossstarting capacitor C1 at a level which is lower than the thresholdvoltage of threshold element D2. Any charge developed across startingcapacitor C1 during this period is continuously discharged to circuitground through diode D1, the collector-emitter junction of transistor Q1and emitter resistor R6. In addition, the time constant of R1 and C1 isselected to be longer than the reciprocal of the operating frequency ofthe oscillator so that capacitor C1 will not recharge through resistorR1 to a level to retrigger diac D2.

When AC input power to the circuit is removed, starting capacitor C1 isunable to receive energy from filter capacitor C6 since the input to thestarting circuit (i.e., the upper end of resistor R1) is connected toone of the input terminals IN2. As a result, current from the filtercapacitor will be unable to flow through the conducting transistors andload circuit to otherwise cause the lamp to momentarily blink or flickerafter the lamp has extinguished.

It is noted that capacitor C1 charges only when the AC input voltage oninput terminal IN2 is positive with respect to input terminal IN1.During this half cycle of the AC supply, current flows from inputterminal IN2, through fuse F1, resistor R1, capacitor C1, diode leg D3A(of diode rectifier bridge D3) to input terminal IN1. No charge path forcapacitor C1 is provided when the AC input voltage on input terminal IN2is negative with respect to input terminal IN1. In addition topreventing this momentary blink or flicker discussed above, the powerdissipated by resistor R1 is reduced since resistor R1 only sees 60 Hzhalf wave voltage.

Upon an end-of-life condition caused by the depletion of emissivematerial on one or both of the lamp filament electrodes (if theelectrodes and lamp envelope remain intact), the lamp acts as an opencircuit element. Under such conditions, the circuit will then run in aseries resonant mode with resonant elements of inductor L1 and capacitorC4. A similar series resonant mode will occur, for example, if the lampis disconnected from a load circuit wherein capacitor C4 is directlyconnected to terminal 3 of inductor L1 and to capacitor C8 (at thejunction of capacitors C9 and C10).

By the nature of any series resonant circuit, the combined impedance ofinductor L1 and capacitor C4 is zero. The only noticeable impedance inthe circuit is the emitter resistor, the winding resistance of inductorL1 and the collector-emitter resistance. The combination of theseresistances is very small. Basically, the circuit is in a short circuitmode. Consequently, the short circuit current of transistors Q1 and Q2will be very high.

Upon detection of an abnormal lamp condition as discussed above, theincreased current through winding W1, tank inductor L1, and tankcapacitor C4 produces an AC voltage of approximately 10 to 15 voltsacross terminals 3 and 4 of secondary winding W3. This AC voltage isdivided by resistors R7 and R9 before being half-wave rectified by diodeD8. The resulting rectified voltage at the cathode of diode D8 chargescapacitor C7. When the voltage on capacitor C7 reaches the thresholdvoltage V_(th) of the semiconductor switch Q3 (e.g., 3.0 to 4.0 volts),switch Q3 becomes conductive and temporarily shorts the base oftransistor Q1 and the ground through diode D9 and the drain-sourcejunction of semiconductor switch Q3. The base current of transistor Q1is diverted into diode D9 and the drain-source junction of semiconductorswitch Q3. This diverted current approximately follows the followingequation:

    i.sub.d(Q3) =K(v.sub.gs -V.sub.th).sup.2 ;

where K is proportional to the forward transconductance, v_(gs) is thegate-source voltage of transistor Q3 and V_(th) is the threshold voltageof transistor Q3. As a result, the voltage developed across the base oftransistor Q1 and the ground is limited to the voltage drop produced bythe series combination of diode D9 and the drain-source junction ofsemiconductor switch Q3.

Because of the limited voltage on the base of transistor Q1 and theground, the base drive current will be insufficient to turn ontransistor Q1, causing an interruption in operation of the oscillator.Thereafter, the voltage on secondary winding W3 drops to zero and thevoltage across capacitor C7 begins to discharge through dischargeresistor R8. When the voltage across capacitor C7 drops below thethreshold voltage of semiconductor switch Q3, switch Q3 becomesnon-conductive and the diode shunt across the base of transistor Q1 andthe ground is effectively removed. Consequently, conduction of thetransistors Q1 and Q2 is resumed.

The amount of time the oscillator is on, and hence the duty cycle duringthis mode of operation, is determined by the proper selection ofresistors R7, R8, R9, capacitor C7 and the forward transconductanceg_(fs) and the threshold voltage V_(th) of transistor Q3. The chargingtime constant established by resistors R7, R9, capacitor C7 and thethreshold voltage V_(th) of transistor Q3 determines the oscillatorinitial starting delay. The forward transconductance of transistor Q3and the charging time constant established by resistors R7, R9 andcapacitor C7 determines the on-time while the discharging time constantof resistor R8 and capacitor C7 determines the oscillator off-time.Preferably, the values of resistors R7, R8, R9, capacitor C7 and thecharacteristics of transistor Q3 are chosen so that the respective timeconstants cause the oscillator to be off substantially the entire dutycycle. In a preferred embodiment, the oscillator during this mode ofoperation is on for approximately 200 milliseconds and off for 10.0seconds. This results in a duty cycle of 2 percent.

Upon replacement of lamp LP1, the oscillator will return to full poweroperation since the voltage developed across winding W3 is insufficientto trigger semiconductor switch Q3 through voltage divider resistors R7and R9.

Although the preferred embodiment teaches the creation of a diode shuntacross the base of transistor Q1 and the ground, it will be apparent tothose skilled in the art that other alternative methods can be utilizedwithout departing from the scope of the invention. For example, it ispossible to interrupt base drive to transistors Q2 by shunting the baseof transistor Q2 while sensing winding W2 instead of shunting the baseof transistor Q1 while sensing winding W3.

As a specific example but in no way to be construed as a limitation, thefollowing components are appropriate to an embodiment of the presentdisclosure, as illustrated by the FIGURE:

    ______________________________________                                        Item Description                                                                              Value or Part No.                                             ______________________________________                                        C1    Capacitor   0.047MFD, 50VDC, SMD                                        C2    Capacitor   0.047MFD, 50VDC, SMD                                        C3    Capacitor   0.047MFD, 50VDC, SMD                                        C4    Capacitor   0.01MFD, 1000VDC                                            C5    Capacitor   0.022MFD, 250VAC                                            C6    Capacitor   47 MFD, 200VDC                                              C7    Capacitor   10 MFD, 25VDC, SMD                                          C8    Capacitor   2200PFD, 250VDC/160VAC                                      C9    Capacitor   0.22MFD, 160VDC/100VAC                                      C10   Capacitor   0.22MFD, 160VDC/100VAC                                      D1    Diode       BYD17J                                                      D2    Diac        2A, 32V (BR100/03)                                          D3    Bridge      BYD17J (4)                                                  D4    Diode       DL4933                                                      D5    Diode       DL4738A                                                     D6    Diode       DL4933                                                      D7    Diode       DL4738A                                                     D8    Diode       DL4148                                                      D9    Diode       SGL41-40                                                    R1    Resistor    510 Kohm, 1/4W, SMD                                         R2    Resistor    20 ohm, 1/4W Fusible (Philips NFR25)                        R3    Resistor    20 ohm, 1/4W Fusible (Philips NFR25)                        R5    Resistor    1.3 ohm, 1/4W Fusible (Philips NFR25)                       R6    Resistor    1.3 ohm, 1/4W Fusible (Philips NFR25)                       R7    Resistor    11 Kohm, 1/4W, SMD                                          R8    Resistor    10 Mohm, 1/4W, SMD                                          R9    Resistor    11 Kohm, 1/4W, SMD                                          Q1    Transistor  5.0A, 850V (BUV46)                                          Q2    Transistor  5.0A, 850V (BUV46)                                          Q3    Transistor  IRFR010, (50V, 0.2 ohm)                                     L1    Inductor    1.2MH                                                       L2    Inductor    1.0MH, 200MA                                                T1    Transformer 6 turns prim. 5 turns each sec.                                   (Base Drive)                                                            RV1   MOV         150VAC, 500A 240CH8, SMD                                    F1    Fuse        2.5A, PICO II                                               LP    Lamp        F13DTT                                                      ______________________________________                                    

There has thus been shown and described a circuit for starting andoperating an arc discharge lamp. The invention does not require avoltage limiting device in the oscillator disabling circuitry and doesnot require removal of the power line voltage to replace a failed lamp.In addition, the circuit remains fully operable upon failure of the lampand does not require relatively expensive, high voltage components inthe oscillator disabling circuitry. Moreover, the circuit does not causean unnecessary use of energy due to continued oscillator operation uponan end of lamp life condition caused by the depletion of emissivematerial on one of the lamp filament electrodes.

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention.Therefore, the aim in the appended claims is to cover all such changesand modifications as fall within the true spirit and scope of theinvention. The matter set forth in the foregoing description andaccompanying drawings is offered by way of illustration only and not asa limitation. The actual scope of the invention is intended to bedefined in the following claims when viewed in their proper perspectivebased on the prior art.

What is claimed is:
 1. A starting and operating circuit for a discharge lamp comprising:a pair of AC input terminals adapted to receive an AC signal from an AC power supply; DC power supply means coupled to said AC input terminals for generating a DC voltage; oscillator means coupled to said DC power supply means to receive said DC voltage and including at least one semiconductor switch and means for driving said semiconductor switch, said driving means including a transformer having a primary winding and a secondary winding, said secondary winding being coupled to said semiconductor switch of said oscillator means; load means coupled to the output of said oscillator means comprising a series combination of said primary winding of said transformer and a tank circuit including a tank inductor and a tank capacitor, said tank circuit having a resonant mode condition; means for connecting a discharge lamp in parallel with said tank capacitor; and oscillator sensing and controlling means having an input connected directly across said secondary winding of said transformer for sensing the output of said oscillator means and for reducing the output of said oscillator means after a predetermined time delay following the initiation of said resonant mode condition of said tank circuit, said oscillator sensing and controlling means including means for shunting drive current to said at least one semiconductor switch of said oscillator means.
 2. The starting and operating circuit of claim 1 wherein said oscillator sensing and controlling means includes a series combination of a diode and a semiconductor switch means coupled across the input terminal of said at least one semiconductor switch.
 3. The starting and operating circuit of claim 2 further including means for controlling the duty cycle of said oscillator means following the initiation of said resonant mode condition of said tank circuit.
 4. The starting and operating circuit of claim 3 wherein said oscillator sensing and controlling means further includes voltage rectifying means in series with charging resistor means.
 5. The starting and operating circuit of claim 1 wherein said predetermined time delay is approximately 200 milliseconds.
 6. An arrangement comprising:a pair of AC input terminals adapted to receive an AC signal from an AC power supply; DC power supply means coupled to said AC input terminals for generating a DC voltage; oscillator means coupled to said DC power supply means to receive said DC voltage and including a pair of semiconductor switches and means for driving said semiconductor switches, said driving means including a transformer having a primary winding and first and second secondary windings coupled respectively to said semiconductor switches; load means coupled to the output of said oscillator means comprising a series combination of said primary winding of said transformer and a tank circuit including a tank inductor and a tank capacitor, said tank circuit having a resonant mode condition; a discharge lamp having lamp terminals; means for connecting said lamp terminals in parallel with said tank capacitor; and oscillator sensing and controlling means having an input connected directly across one of said secondary windings of said transformer and an output coupled to one of said pair of semiconductor switches of said oscillator means for reducing the output of said oscillator means after a predetermined time delay following the initiation of said resonant mode condition of said tank circuit, said oscillator sensing and controlling means including means for shunting drive current to said one of said pair of semiconductor switches of said oscillator means.
 7. The arrangement of claim 6 wherein said oscillator sensing and controlling means includes a diode and a semiconductor switch means coupled across the base terminal of one of said semiconductor switches.
 8. The arrangement of claim 7 further including means for controlling the duty cycle of said oscillator means following the initiation of said resonant mode condition of said tank circuit.
 9. The arrangement of claim 8 wherein said oscillator sensing and controlling means further includes voltage rectifying means in series with charging resistor means.
 10. The arrangement of claim 7 wherein said predetermined time delay is approximately 200 milliseconds. 